At lower temperature, sticking in the die land and the sudden building up of pressure and then breakaway will cause transverse cracking.
Extrusion is the process by which a
block/billet of metal is reduced in cross
section by forcing it to flow through a die
under high pressure.
It is a manufacturing process, in which a
block of metal enclosed in a container is
forced to flow through the opening of a
• The metal is subjected to plastic
deformation & it undergoes reduction
In general, extrusion is used to produce cylindrical bars
or hollow tubes or for the starting stock for drawn rod,
cold extrusion or forged products.
Most metals are hot extruded due to large amount of
forces required in extrusion. Complex shape can be
extruded from the more readily extrudable metals such
The products obtained are also called extrusion.
What Is Extrusion
CLASSIFICATION OF EXTRUSION PROCESSES
Hot Cold Horizontal Vertical
It is a process in which the metal billet, placed
in a container is forced by a ram to pass
through a die.
In this type the direction of flow of metal is,
in same as that of movement of ram.
The punch closely fits the die cavity to
prevent backward flow of the material.
The dummy block or pressure plate, is placed at
the end of the ram in contact with the billet.
Friction is at the die and container wall
requires higher pressure than indirect
To repeatedly transmit the force of the ram, at high
temperature, to the alloy.
To expand quickly under load and maintain a secure seal
with the container wall.
To separate cleanly from the billet at the end of the stroke.
To cause no gas entrapment that can result in blistering, or
damage the face of the container and/or dummy block.
To compensate for minor press misalignment.
To be quickly and easily removed and replaced.
To function effectively until a production run is complete.
Function Of Dummy Block
It is a process in which a hollow ram
containing the die is forced into the
container, containing metal.
Hollow ram limits the applied load.
The movement of metal is opposite to
the direction of ram motion.
The hollow ram containing the die is
kept stationary and the container with
the billet is caused to move.
Friction at the die only (no relative
movement at the container wall)
requires roughly constant pressure.
Hollow ram limits the applied load.
It is done at fairly high temperatures, approximately
50 to 75% of the melting point of the metal.
Die life and components are effected due to the high
temperatures and pressures, which makes lubrication
necessary. Pressures Ranges: 35-700 Mpa.
Hot extrusion is a hot working process, which means it
is done above the material's recrystallization
temperature to keep the material workable hardening
and to make it easier to push the material.
The biggest disadvantage of this process is its cost for
TYPES OF HOT EXTRUSION
Before the billet is inserted into the hot
extrusion container, a suitable lubricating
system is positioned immediately ahead of
the die in order to reduce frictional
Oil and graphite are used at lower
temperature whereas at higher
temperatures, glass powder is used.
Copper alloys, titanium alloys, alloy steels,
stainless steels, and tool steels are
extruded using lubrication.
LUBRICATED HOT EXTRUSION
No lubrication is used on the billet,
container, or die for reducing
It has the ability to produce very
complex sections with excellent
surface finishes and low dimensional
Solid and hollow dies with flat shear
faces are typically used.
NON-LUBRICATED HOT EXTRUSION
• In the hydrostatic extrusion process the billet is
completely surrounded by a pressurized liquid,
except where the billet contacts the die.
• The rate, with which the billet moves when
pressing in the direction of the die, is thus not
equal to the ram speed, but is proportional to
the displaced hydrostatics medium volume.
• The process must be carried out in a sealed
cylinder to contain the hydrostatic medium.
• Pressure = 1400 Mpa (approx.)
Advantages & Limitations
• Elimination of large friction force
between the billet and the
• Possible to use dies with a very low
semi cone angle (α-200).
• This process can be done hot, warm,
or cold, however the temperature
is limited by the stability of the fluid
•Not suitable for hot-working due
to pressurized liquid.
•A practical limit on fluid pressure is
around 1.7GPa currently exists
because of the strength of the
•The liquid should not solidify at
high pressure this limits the
Complex solid or hollow
shapes can be produced.
Small quantities can be
Delivery times are often far
shorter than alternative
High equipment set up and
Extrusion process for metals is at very
Die is preheated to increase its life, so
there are chances of oxidation of hot
Process Wastage is higher as
compared to rolling.
Cold extrusion is the process done at room
temperature or slightly elevated temperatures.
This process can be used for materials that can
withstand the stresses created by extrusion.
Cold extrusion is done at room temperature or
near room temperature. The advantages of this
over hot extrusion are the lack of oxidation,
higher strength due to cold working, closer
tolerances, good surface finishing.
Examples of products produced by this process
are: collapsible tubes, fire extinguisher cases, and
shock absorber cylinders.
Good control of dimensional
Improved surface finish.
No need for heating billet.
No oxidation takes place.
Tooling cost is high, therefore
large production lot size is
Special coating is required to
reduce friction and to maintain a
lubricant film throughout.
Limited deformation can be
Cu, Pb, Sn, Al Alloys, Ti, Mo, V, Steel, Zr parts
can be extruded.
Tubes, Gear Blanks, Aluminum Cans,
Cylinders, Fire Extinguisher Cases, Shock
Absorber Cylinders, and Automotive Pistons
Application Of Cold Extrusion
• A manufacturing process in which a
small shot of solid material is placed
in the die and is impacted by a ram,
which causes cold flow in the
• It may be either direct or indirect
extrusion and it is usually performed
on a high- speed mechanical press.
Although the process is generally
performed cold, considerable heating
results from the high speed
A short lengths of hollow shapes,
such as collapsible toothpaste tubes or
Requires soft materials such as Al, Pb,
Cu or Sn.
The layout of operating
machine is horizontal and
movement of billet as well
as of ram is horizontal in
15-50 MN capacity.
It is mostly used for
commercial extrusion of
bars and shapes.
Horizontal extrusion process
Vertical extrusion process
• The movement of billet and ram is vertical in orientation.
• 3-20 MN capacity.
• Mainly used in the production of thin-wall tubing.
Commonly used materials are Tool Steels and Carbides
Diamond dies are used for fine wire.
For improved wear resistance, steel dies may be chromium
plated, and carbide dies may be coated with titanium nitride
For Hot drawing, cast-steel dies are used .
Dies are made from highly alloy tools steels or ceramics
Heat treatments such as nitriding are required (several
times) to increase hardness (1000-1100 Hvor 65-70 HRC).
This improves die life.
Proper lubrication is essential in extruding, in order to
improve die life, reduce drawing forces and
temperature, and improve surface finish.
Types of Lubrication-
a) Wet : Dies and Rods are completely immersed in
b) Dry : Surface of the rod to be drawn is coated with a
c) Coating : Rod or Wire is coated with a soft metal that
acts as a solid lubricant.
d) Ultrasonic Vibration : of the dies and mandrels.
FACTORS AFFECTING THE EXTRUSION FORCE
1. Type of extrusion
2. Extrusion ratio
3. Working temperature
5. Frictional conditions at the die and the container wall
1. Extrusion ratio, R -
It is defined as the ratio of the initial cross-sectional area ,
Ao, of the billet to the final cross-sectional area , Af, after
R ~ 40:1 for hot extrusion of steels.
R ~ 400:1 for aluminum.
2.Fractional reduction in area, r -
3. Velocity -
Velocity of extruded product = (Ram velocity) x (R)
4. Extrusion force -
where k = extrusion constant,
Inhomogeneous deformation -
In direct extrusion, a dead zone along the outer surface
of the billet due to the movement of metal in the centre being
higher than the periphery.
After 2/3 of the billet is extruded, the outer surface of the
billet (normally with oxidised skin) moves toward the centre
and extrudes to the through the die, resulting in internal oxide
It is the ranging from a badly roughened surface to
repetitive transverse cracking called fir-tree cracking.
It is due to longitudinal tensile stresses generated as the
extrusion passes through the die.
In hot extrusion, this form of cracking usually is
intergranular and is associated with hot shortness.
It can occur at low
extrusion ratio due to
low frictional conditions
on the zone of
deformation at the
High friction(at a the
Low friction centre burst.
Centre burst or chevron cracking
Structure & properties are varied within the extrusions due to
non-uniform deformation for example at the front and the
back of the extrusion in both longitudinal and transverse
Regions of exaggerated
grain growth, due to high
hot working temperature.
Variations in structure and properties
(in aluminium extrusion)
High temperatures generated cause incipient
melting, which causes cracking.